FIELD OF THE INVENTION
This invention relates to a data decoding device for decoding an RF playback signal read out from a recording medium having recorded thereon the information using the run-length-limited code (RLL code) based on at least a comparison level for outputting channel bit data. More particularly, it relates to a data decoding method and a data decoding device in which, if there is a portion not satisfying the requirement for minimum run length (minimum continuation length) or maximum run length (minimum continuation length) of the same symbol in channel bit data, bits with high probability of bit error are selected based on the playback RF signal level at the time of level decision and the bits so selected are corrected for outputting channel bit data satisfying the requirement for minimum run length or maximum run length.
When transmitting data or when recording data on a recording medium, such as a magnetic disc, optical disc or a magneto-optical disc, data are modulated for suitability to transmission or recording. Among the techniques for modulation is a block code in which a data string is blocked into units made up of m.times.i bits, termed data words, which are then converted into code words of n.times.i bits in accordance with a suitable coding rule. If i=1, the resulting codes are fixed-length codes, whereas, if i can be selected from plural numbers, that if it is 2 or more and if imax (maximum value of i)=r is used for conversion, the code is the variable length code. The codes represented by the block coding are variable-length code (d, k; m, n; r), where i is the constraint length, r is the maximum constraint length and d and k denote the minimum run value and the maximum run value of `0`s comprised between neighboring `1`s in the code string, respectively.
As an illustrative example, the modulation system for a compact disc (CD) system is explained. The compact disc system employs an eight to fourteen modulation (EFM). After pattern conversion of 8-bit data words into 14-bit code words (channel bits), 3 merging bits are appended for decreasing dc components following EFM. The resulting data is recorded on the disc by NRZI. The EFM and appendage of the merging bits are carried out so that the conditions of the minimum run value of the logical level `0` of 2 and the maximum run value of the logical level `0`s will be met. Therefore, the parameters of this modulation system are (2, 10; 8, 17; 1). If the bit length of the channel bits (recording waveform string) is T, the minimum length between transitions Tmin is 3 (=2+1)T. If the data length T data of the data string is Tdata, the window margin Tw is (m/n) X Tdata, its value being 0.47 (=8/17)T data.
After NRZI modulation, the minimum run length d' of the same symbol is d'=d+1=2+1=3, with the maximum run length being k'=k+1=10+1=11.
With the above compact disc system, if the pits are contracted on the optical disc, the recording density can be enhanced, in which case the minimum pit length corresponding to the minimum length between transition lengths Tmin is decreased. If this minimum pit length becomes excessively shorter than the spot size of the laser beam, the pits become difficult to detect, so that errors tend to be produced.
If, in the disc reproduction, skew is applied to the disc display surface, the error rate is worsened. The disc skew is classified into that in a tangential direction and that in a radial direction of the optical disc with respect to a plane of the objective lens normal to the optical axis of the lens. Of these, the skew in the tangential direction worsens the error rate relatively promptly. These skews lead to margin decrease in system designing.
The distribution of errors in the length of continuation of the same symbol has been checked with respect to the respective directions of the skew. The error with respect to the skew in the tangential direction mainly occurs in case of a shorter length of continuation of the same symbol. Thus, it has been found that the error rate has become worsened as a result of decoding the length of Tmin(d') to a length of Tmin-1(d'-1). It has also been found that if, in the above EFM modulation system, skew is produced in the tangential direction, there frequently occurs an error due to decoding of the minimum length between transitions Tmin of 3T into 2T, where T is the bit length in the recording waveform string.
On the other hand, some margin is allowed in disc asymmetry in disc manufacture such that it is necessary to take into account the fact that the playback waveform tends to become vertically asymmetrical with respect to the center level.
Among known correction methods by signal processing against worsened error rate is a viterbi decoding method. The viterbi decoding method, as one of the maximum likelihood decoding methods for reducing the encoding errors for retrieving the shortest geometrical distance, is to discard the less probable way and simplifying the retrieval of more likely values in decoding. With the viterbi decoding, it is possible to add an internal algorithm of compensating the minimum length between transitions Tmin.
However, the viterbi decoding method has a drawback that the memory length is large and the memory circuit becomes complex to increase the hardware scale. Moreover, with the viterbi decoding method, asymmetry needs to be removed, such that, in a system in which asymmetry is not allowed, such as optical disc, the circuit needs to be optimized with respect to asymmetry, thus further complicating the structure.
Thus, in a recording medium, such as an optical disc, there are occasions wherein the skew margin cannot be secured without difficulties. In particular, the skew margin is decreased in the tangential direction.
In addition, in the recording medium of high recording density, such as optical disc, the minimum length between transitions Tmin becomes difficult to reproduce in stability, thus lowering the error rate.